1. Technical Field
The present invention relates to a magnetic field sensor that uses a magnetic impedance effect.
2. Background Art
For example, JP-A-H09-127218, JP-A-2000-180521, and JP-A-2015-92144 disclose a related art of a magnetic sensor employing a magnetic detection element (MI element) using a magnetic impedance effect. For example, in a high permeability alloy magnetic material such as an amorphous alloy wire, impedance sensitively changes in response to an external magnetic field due to the influence of a skin effect. This is a magnetic impedance effect.
In a configuration disclosed in JP-A-H09-127218, an MI element is incorporated in a Colpitts oscillation circuit. When an AC bias current flows to a coil wound around the MI element, an AC bias magnetic field is applied to the MI element. Then, an amplitude modulated waveform is obtained in the output of the oscillation circuit by a change in impedance of the MI element in response to the external magnetic field and the bias magnetic field. A height difference of the amplitude modulated waveform corresponds to the strength of the external magnetic field. Thus, an output signal having a pulse width modulated digital waveform is obtained in such a manner that the amplitude modulated waveform is detected, a DC element is removed, and a voltage is compared by a comparator. That is, the strength of the external magnetic field is obtained from a change in amplitude of the output of the oscillation circuit.
In a configuration disclosed in JP-A-2000-180521, a high-frequency sine waveform current output from the oscillation circuit is applied to both ends of a magnetic core of a thin film magnetic impedance element via a buffer circuit. A detector circuit detects a magnetic change amount of the external magnetic field from a change amount of the high-frequency current changing in response to the external magnetic field applied to the magnetic impedance element. A hysteresis cancellation circuit that removes the hysteresis of the magnetic impedance element is provided. In order to move the operation point of the magnetic impedance element, a current flows to the bias coil. Furthermore, a current flows to a negative feedback coil in response to the detected magnetic field.
In the magnetic field sensor disclosed in JP-A-2015-92144, a magnetic impedance element having a specific configuration is employed. That is, the magnetic impedance element has magnetic anisotropy in which a longitudinal direction is set as a magnetic field detection direction and an easy magnetization axis of a magnetic film is set as the longitudinal direction. Here, since the magnetic field detection direction is the same as the easy magnetization axis direction of the magnetic film, it is possible to exhibit a magnetic impedance characteristic of a pyramid shape. For that reason, it is not necessary to apply DC and AC biases to a point where an inclination becomes steep like an M-shaped characteristic.
Furthermore, since the hysteresis in the pyramid shape is smaller than that of the M shape, the detection accuracy can be improved. Meanwhile, since the inclination is inclined by a predetermined degree over the entire area, the detection range can be widened. Thus, the consuming current can be suppressed, the detection accuracy can be improved, and the detection range can be widened.
However, the magnetic field sensor of the related art using the magnetic impedance effect disclosed in JP-A-H09-127218 and JP-A-2000-180521 has the following problems. (1) A range of detecting the magnetic field is narrow. (2) Since the magnetic impedance element has the magnetic impedance characteristic of an M shape, it is not possible to perform a highly sensitive measurement if the AC bias is not applied to a position where the inclination is steep in the case of using the AC bias. For that reason, the consuming current increases. (3) In the magnetic impedance element having the magnetic impedance characteristic of an M shape, the hysteresis becomes larger than that of the pyramid shape. For that reason, the detection accuracy is degraded.
Meanwhile, when the magnetic impedance element having the pyramid-shaped magnetic impedance characteristic disclosed in JP-A-2015-92144 is employed, the problems (2) and (3) can be solved. However, when the magnetic field is detected by using a circuit having a configuration illustrated in FIG. 1 of JP-A-2015-92144, the pulse necessary for the output of differentiating circuit illustrated in FIG. 6D of JP-A-2015-92144 does not occur if the external magnetic field increases. For this reason, the phase of the pulse cannot be detected and the magnetic field cannot be detected. Thus, the problem (1) cannot be solved.
The invention has been made in view of such circumstances and an object of the invention is to provide a magnetic field sensor capable of suppressing a current from consuming, improving detection accuracy, and widening a magnetic field detection range.